Dual-templated 3D nitrogen-enriched hierarchical porous carbon aerogels with interconnected carbon nanosheets from self-assembly natural biopolymer gel for supercapacitors

Electrochimica Acta 353 (2020) 136514

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Dual-templated 3D nitrogen-enriched hierarchical porous carbon aerogels with interconnected carbon nanosheets from self-assembly natural biopolymer gel for supercapacitors Panyu Li , Hongyang Xie , Yali Liu , Jie Wang , Xuqian Wang , Yi Xie , Wanrong Hu , Tonghui Xie , Yabo Wang , Yongkui Zhang * Department of Pharmaceutical & Biological Engineering, School of Chemical Engineering, Sichuan University, Chengdu, Sichuan, 610065, PR China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 13 December 2019 Received in revised form 3 May 2020 Accepted 24 May 2020 Available online 28 May 2020

This work reports the design and fabrication of nitrogen-enriched hierarchical porous carbon aerogel (NPCA) with high supercapacitance performance derived from self-assembly natural biopolymer gel using a novel dual-template method. The as-obtained NPCA exhibited a honeycomb-like 3D network architecture composed of interconnected carbon nanosheets with hierarchical porous structure, large specific surface area (SSA, large to ~1438 cm3 g
1) and high content of N element (large to ~6%). The 3D hierarchical porous structure was designed and tailored to enhance the electron/ion transport ability. NaCl was in favor of the enhancement of SSA/graphitization degree and the support of 3D architecture, while NaOH played an important role in the formation of micropores. The N dopants were introduced to provide the extra pseudocapacitance, which could improve the performance of energy storage. It was also found that the SSA, graphitization degree and N dopants were highly affected by pyrolysis temperature. Among the resultant samples, NPCA-650 displayed the highest specific capacitance (264.3 F g
1 at 0.5 A g
1) attributed to not only the developed pore structure but also the abundant active N dopants. The as-assembled symmetric supercapacitor exhibited a high energy density of 12.4 Wh kg
1 with excellent cycling stability. Overall, this work provides a novel approach for the fabrication of low-cost biomass-based energy storage materials and could be also helpful for the design and tailoring of the hierarchical porous carbon aerogel architecture. © 2020 Elsevier Ltd. All rights reserved.

Keywords: Hierarchical porous carbon aerogels Natural biopolymer Energy storage Dual templates Nitrogen doping

1. Introduction Currently, environmental and energy issues caused by the depletion of fossil fuel have attracted increasing attention. To address these issues, the developments of eco-friendly and effective energy storage devices are greatly important. Among the various devices, supercapacitor is a promising one because of its high power density, excellent charge/discharge performance, long cycle life and good product safety [1,2], which has extensive application prospects in portable laptops, cell phones and electrical

* Corresponding author. Sichuan University, No. 24 South Section 1, Yihuan Road, Chengdu, 610065, PR China. E-mail addresses: [email protected] (P. Li), [email protected] (H. Xie), [email protected] (Y. Liu), [email protected] (J. Wang), [email protected] (X. Wang), [email protected] (Y. Xie), [email protected] (W. Hu), [email protected] (T. Xie), [email protected] (Y. Wang), zhangyongkui@ scu.edu.cn (Y. Zhang). https://doi.org/10.1016/j.electacta.2020.136514 0013-4686/© 2020 Elsevier Ltd. All rights reserved.

vehicles etc. [3]. Although supercapacitors have some advantages, the practical applications are usually limited by their low energy densities [4]. According to the equation of E ¼ 1=2 CV 2 , the energy density could be enhanced by extending the operating voltage window or improving the capacitance of electrode. To enlarge the voltage window, various strategies have been applied. For example, asymmetric configurations are expected to exhibit wider voltage windows than symmetric supercapacitors [5]. The improvement of electrolyte system is another route. Compared to the traditional alkali and acid electrolytes, neutral electrolytes (e.g., Na2SO4) and non-aqueous electrolytes, especially ionic liquids electrolytes, could enlarge voltage windows from about 0e1.0 V to 0e1.6/1.8 V and 0e4.0 V, respectively [6e8]. The aqueous solutions of acidic ionic liquids as electrolyte were also reported to improve the cycling stability of supercapacitors [9]. Besides the enlargement of voltage windows, the increase of capacitance is also important to enhance the energy density.

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Electrode material is a crucial factor affecting the capacitance. Porous carbon materials, such as carbon aerogels (CAs), are widely used as the electrodes for EDLCs due to the good electronic conductivity, high specific surface area, good chemical stability, low cost and abundant sources [10,11]. Generally, there are two energy storage mechanisms for supercapacitors, namely electrical double layer capacitors (EDLC) and pseudo-capacitors [12]. EDLCs store energy through electrostatic charge accumulated in the double layer at the interface between electrode materials and electrolyte, while pseudo-capacitors achieve their energy storage capacity via faradaic redox reactions at the surface of electrode materials [2,13]. Traditional carbons achieve energy storage mainly through EDLC approach but with low capacity [14]. To address this issue, carbons were usually combined with pseudocapacitance-like materials such as metal oxides and layered double hydroxides [15e17]. Besides metallic compounds, the heteroatoms, especially nitrogen, are usually introduced into carbon framework to improve the capacitive performance by enhance the wettability and provide extra pseudocapacitance [10,18]. Besides heteroatom doping, the pore structure is also important for the performance improvement of carbons. It has been proved that 3D hierarchically porous structure could be in favor of the enhancement of energy storage performance due to the excellent electron/ion transport ability [19]. Traditional activation methods for porous structure could suffer from some drawbacks, for example, being not cost-efficient and eco-friendly because of the use of large amount of harmful chemicals, high operation temperature and multi-step purification procedures [20,21]. From the friendly environment point of view, a green method to fabricate porous carbon is desired. Recently, NaCl has been used as a green template for the construction of porous carbon with 3D structure [22,23]. NaCl could be removed by water washing without destroying the carbon structure and recycled by a simple recrystallization treatment [21]. To enhance the specific surface area and form a hierarchical porous structure, a splash of etching agents (e.g. KOH or NaOH) could be added to introduce more nanopores, which may result in a higher capacitance for electrochemical double-layer capacitors (EDLCs) [24]. The combination of NaCl and a splash of etching agents could not only form a 3D hierarchical porous structure but also mitigate the threat to environment compared to the single KOH or NaOH activation method. Natural biomass has shown its superiorities as the raw material for carbon preparation due to its low cost, sustainability and renewability [25,26]. Biopolymers, such as carbohydrates and proteins are widespread in the nature as the main compositions of organisms. Due to the abundant reserves and the renewability, biopolymers could be promising precursors for carbon preparation. Up to now, many biopolymers (e.g. cellulose, starch and sericin) have been used to prepare carbons or carbon-based materials for various applications, such as supercapacitors, removal of pollutants, and electrochemical sensors [27e30]. Xanthan gum is a polysaccharide produced via fermentation by Xanthomonas campesteris using various organics as the substrates, such as glucose, sucrose and a variety of biomass wastes [31e33]. In the previous work, the authors used kitchen waste as the substrate for xanthan production [34]. The waste biomass-based xanthan gum could be a low-cost and renewable carbon source for carbon preparation. The production of advanced materials from waste biomass-based xanthan gum could also enhance economic efficiency of the kitchen waste treatment. In addition, soy protein, with high protein level, could be a model nitrogen source to introduce N element into carbons [35]. Meanwhile, a gel could be easily formed by selfassembly of the two biopolymers due to their unique structure for further preparation of CA with 3D architecture [36]. As far as we know, there is little literature on the synthesis of CA from waste

biomass-based xanthan gum using NaCleNaOH dual-template method for supercapacitors. In the present work, waste biomass-based xanthan gum and soy protein were used as the carbon and nitrogen sources to prepare nitrogen-doped porous carbon aerogels (NPCAs). Xanthan-soy protein gel was self-assembled of the two biopolymers with the presence of templates (NaCl and a splash of NaOH). The carbon aerogels were then obtained via a freeze drying-pyrolysis treatment. When used as the supercapacitor electrode material, the obtained NPCA exhibited a high specific capacitance, energy density and excellent cycling stability due to its 3D hierarchical porous structure and abundant active N dopants. 2. Experimental 2.1. Materials Xanthan gum (XG, purity of 96%) was produced by fermentation of kitchen waste as described in our previous work [34]. Soy protein isolate (SPI, food grade, purity of 99%) was purchased from Hunan Century Huaxing Bioengineering Co., Ltd. (Hunan, China). Sodium chloride and sodium hydroxide (analytical grade, purity of 99.5%) were purchased from Chengdu Kelong Chemicals Co., Ltd. (Sichuan, China). 2.2. CA preparation The synthesis process of 3D nitrogen-doped porous carbon aerogels (NPCAs) is shown in Fig. 1a. Xanthan gum (1 g) and SPI (0.8 g) were added into 50 mL deionized water containing 5 g NaCl and 0.2 g NaOH, after which the mixture was stirred thoroughly at 95  C for 4 h. After stopping stirring, the mixture was kept at 95  C for 1 h and then at ambient temperature for 12 h. The obtained gellike hybrid was then subjected to freeze-drying process. Subsequently, the dried precursor was pyrolytically treated in a tubular furnace under argon atmosphere at desired temperature values for 1 h. The black mixture was washed with deionized water until the pH reached 6.5e7.0. The obtained samples were denoted as NPCA-x (x: carbonization temperature, e.g. 500, 650 and 820  C). For comparison, the samples were also prepared without NaOH or both templates and carbonized at 650  C, which were denoted as NPCANaCl-650 and NPCA-n-650, respectively. 2.3. Characterization of samples Scanning electron microscopy (SEM, JSM-7500F) was performed to observe the morphology of the samples. X-ray diffraction (XRD) was performed on a Rigaku D/max-TTR III X-ray diffractometer with Cu Ka radiation. Raman spectroscopy was conducted on an Andor SR-500i spectroscope with 532 nm laser excitation. Elemental analysis was performed on an elemental analyzer (Elementar Vario EL cube). X-ray photoelectron spectroscopy (XPS) was conducted on a Thermo Scientific Escalab 250Xi spectrometer. N2 adsorption/desorption isotherms were acquired at
196  C on a Micromeritics ASAP 2020 sorption analyzer. The specific surface area (SSA) and pore size distribution (PSD) were obtained by Brunner-Emmet-Teller (BET) and non-local density functional theory (NLDFT) methods. 2.4. Electrochemical characterization The working electrodes for electrochemical measurements were obtained by mixing the obtained NPCAs, carbon black and polyvinylidene fluoride in a mass ratio of 8:1:1. The mixture was spread onto Ni foam, dried at 60  C and pressed into a sheet under 10 MPa.

P. Li et al. / Electrochimica Acta 353 (2020) 136514

3

Fig. 1. (a) Schematic illustration of the synthesis process of 3D NPCA, (bec) SEM and (dee) TEM images of NPCA-650.

The electrochemical measurements were carried out on a CHI660E electrochemical workstation (Shanghai Chenhua, China). For the three-electrode system, platinum foil, Hg/HgO electrode and 2 M KOH were used as the counter electrode, reference electrode and electrolyte, respectively. For the two-electrode system, two same weight electrodes were separated by a polypropylene diaphragm and 1 M Na2SO4 was used as the electrolyte. Cyclic voltammetry (CV) and galvanostatic charge-discharge (GCD) tests were conducted within a potential window of
1e0 V or 0e1.6 V. Electrochemical impedance spectroscopy (EIS) was performed in a frequency range from 10 mHz to 100 kHz. The specific capacitance and power/energy density were calculated as described in the authors’ previous work [7]. 3. Results and discussion 3.1. Characterization of samples The morphology of the as-obtained NPCAs were first observed by SEM (Fig. 1bec and Fig. S1). The samples prepared with NaCl template exhibited a honeycomb-like 3D network architecture, which was composed of interconnected carbon nanosheets. In comparison, NPCA-n-650 presented an irregular powder-type morphology, which demonstrated the important role of salt templates in the formation of 3D structure of CA. Actually, it has been reported that NaCl crystals could be used as templates to support the carbon structure for 3D architecture [21,37]. When heated to a high temperature, the xanthan-soy protein gel turned into carbon thin layer covering the surface of NaCl crystals and the latter could play a role of skeleton to hinder the pulverization or collapse of the carbonized structure [21,23]. After the pyrolysis process, the in situ formed NaCl crystal templates were easily removed by water, leaving interconnected pores. The microstructure of NPCA-650 was further observed by TEM. The interconnected and overlapped thin crumpled carbon nanosheets with some macropores and large mesopores were confirmed (Fig. 1d). The self-stacked NaCl crystals helped form the large nanopores. Moreover, numerous micropores could be observed in the HRTEM image (Fig. 1e), which could be generated from the redox reaction between NaOH and carbon. The hierarchically porous structure could provide fast channels for ion

transfer and improve the wettability/compatibility of carbons, as well as utilization efficiency of SSA [37]. Furthermore, the graphitic fringes with interlayer spacing of 0.42 nm could be found in Fig. 1e, suggesting the partial graphitization of the as-prepared sample, which would be in favor of the enhancement of electrochemical performance [18]. The pore structure was further investigated by nitrogen adsorptionedesorption isotherms. NPCA-n-650 showed a very low adsorption volume of N2, suggesting a small SSA with a lack of pores (Fig. 2a). The other samples displayed the combined Type I/IV isotherm with a hysteresis, which indicated the presence of abundant micropores and mesopores [38]. The uptake at high relative pressures suggested the existence of macropores, which could derived from the 3D architecture of NPCAs [39]. The PSD of samples are shown in Fig. 2b, which also demonstrated the hierarchical porous structure of NPCAs, as observed in SEM and TEM images. The pore characteristics of NPCAs were listed in Table 1. NPCA820 exhibited the largest SSA/total pore volume and the sample with no templates displayed the smallest SSA/total pore volume (only 14 m2 g
1/0.02 cm3 g
1), demonstrating the salt templates were crucial for pore formation. Compared to NPCA-NaCl-650, NPCA-650 exhibited a much larger SSA and porosity, especially for the increase in micropore volume, suggesting that the introduction of NaOH could effectively improve the SSA by forming more micropores, which would be favor to improve the electrochemical performance [40]. The above results suggested that the application of dual templates was favor to design and construct a hierarchical porous structure based on the function of each template. It was also found that pyrolysis temperature also showed a significant effect on pore structure. The increase of temperature could result in an increase in SSA, as reported in some works [41,42]. Moreover, the yields of the samples with NaOH were lower than the other two samples. Especially, for NPCA-820, the yield was as low as 14%, only about a quarter of that of NPCA-NaCl-650. XRD patterns of NPCAs are shown in Fig. 2c. A broad peak at ~22 and a weak peak at ~43 were observed in the XRD patterns, which are corresponding to (002) and (100) planes of graphitic carbon, indicating the partial graphitization of NPCAs [38,43]. The carbon structure was further investigated by Raman spectroscopy as shown in Fig. 2d. Two obvious peaks at ~1340 cm
1 (D band) and

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P. Li et al. / Electrochimica Acta 353 (2020) 136514

Fig. 2. (a) Nitrogen adsorption-desorption isotherms, (b) pore size distribution, (c) XRD patterns and (d) Raman spectra of NPCAs.

Table 2 Chemical compositions of NPCAs.

Table 1 Pore structure parameters and yields of NPCAs. Sample

SSA/m2 g
1

VT/cm3 g
1

Vmicro/cm3 g
1

Yield/%

ID/IG

NPCA-500 NPCA-650 NPCA-820 NPCA-NaCl-650 NPCA-n-650

218 760 1438 255 14

0.21 0.55 0.94 0.19 0.02

0.11 0.37 0.48 0.14 e

36 27 14 49 45

0.88 0.98 1.04 0.94 1.10

Sample

Ca /wt%

Na /wt%

Ha /wt%

N-6b /at%

N-5b /at%

N-Qb /at%

N-Xb /at%

NPCA-500 NPCA-650 NPCA-820 NPCA-NaCl-650 NPCA-n-650

72.61 75.82 83.84 73.35 68.11

6.04 5.43 4.06 5.48 5.76

3.54 3.20 1.89 2.64 2.16

38.59 28.56 31.30 35.72 39.91

39.31 42.38 32.93 30.10 36.47

14.36 16.29 20.97 24.62 13.83

7.74 12.77 14.80 9.56 9.79

a


1

~1585 cm (G band) were observed. D band indicates the structural defects or edges, while G band represents first-order scattering of the E2g mode observed for sp2-hybridized graphitic carbon [44]. The intensity ratio (ID/IG) of D band and G band is usually used to evaluate the degree of graphitization or disorder [38,41]. As shown in Table 1, the value of ID/IG for NaCleNaOH induced samples slightly increased with the increasing pyrolysis temperature, indicating a more disordered structure, which was similar to the published work [41]. Meanwhile, from the value of ID/IG, it could be found that NPCA-n-650 exhibited a less graphited structure than NPCA-650 and NPCA-NaCl-650, suggesting a very critical role of NaCl in the graphitizing of carbons as reported before [45]. The elemental compositions of NPCAs were determined by EA. As shown in Table 2, the introduction of salt templates did not highly influence the content of N as it was 5.43% for NPCA-650, 5.48% for NPCA-NaCl-650 and 5.76% for NPCA-n-650. However, the content of N could be highly affected by pyrolysis temperature. The excessive temperature was not favor of the retaining and introduction of N element in carbon framework. The surface chemical structure was further investigated by XPS. Fig. 3a shows the XPS survey spectra of NPCAs and C 1s, N 1s and O 1s peaks were detected. It was further demonstrated that N atoms were successfully introduced into carbon framework. The highresolution N 1s XPS spectra are shown in Fig. 3bef. N 1s could be

b

Based on EA measurement, wt%. based on the N 1s XPS analysis, at%.

deconvoluted into four peaks, associating with pyridinic N (N-6), pyrrolic N (N-5), quaternary N (N-Q) and oxidized N (N-X). The corresponding content of the four N species are listed in Table 2. It was obvious that N-6 and N-5 decreased as the pyrolysis temperature increased, while N-Q and N-X showed a positive relationship with pyrolysis temperature. Actually, it has been reported that N-6 and N-5 were not thermally stable and they could be converted to the more stable quaternary N (N-Q) and oxidized to N-X [46]. NPCA-NaCl-650 exhibited a much higher content of N-Q than NPCA-n-650, demonstrating that NaCl could help form graphitic structure. N-6 and N-5 are believed to make a significant contribution to provide additional active sites to enhance the charge storage in supercapacitors [47]. Quaternary N (N-Q), namely graphitic N, substituting one C atom in the graphitic structure, could promote electron transfer inside the carbon structure [48,49].

3.2. Electrochemical performance of NPCAs The prepared NPCAs were used as electrode materials for energy storage. The electrochemical performance was first investigated in a three-electrode system with 2 M KOH as the electrolyte. Fig. 4a

P. Li et al. / Electrochimica Acta 353 (2020) 136514

5

Fig. 3. (a) XPS survey spectra and (bef) high-resolution N 1s XPS spectra of NPCAs.

shows the CV curves of NPCAs at a scan rate of 20 mV s
1 with a voltage window of
1e0 V. All the samples exhibited quasirectangular shaped CV curves with bumps, suggesting a synergetic contribution of double layer capacitance and pseudocapacitance [50]. The heteroatom dopants, N dopants in this study, could account for the introduction of pseudocapacitance. Specifically, N-6 and N-5 are believed to be electrochemically active sites and electron donors to enhance capacitance through following two faradaic reactions [12]:

> C ¼ NH2 þ 2OH
4 > C ¼ NH þ 2e
þ H2 O

(1)

> CNH2 þ 2OH
4 > CNHOH þ 2e
þ H2 O

(2)

In addition, among the five samples, NPCA-650 presented the largest encircled area of CV curve, indicating its highest capacitance. Interestingly, NPCA-820 showed a lower capacitance than NPCA-650 although it exhibited the largest SSA and most developed pore structure. The specific capacitance mainly depended on

Fig. 4. Electrochemical performance of NPCAs in a three-electrode system using 2 M KOH as the electrolyte: (a) CV curves at 20 mV s
1, (b) GCD curves at 0.5A g
1, (c) CV curves of NPCA-650 at various scan rates, (d) GCD curves of NCA-650 at various current densities, (e) specific capacitance at current densities in the range of 0.5e20 A g
1 and (f) cycling stability of NPCA-650 electrode at a current density of 5 A g
1.

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P. Li et al. / Electrochimica Acta 353 (2020) 136514

not only the porosity/SSA but also N dopants. Because of the excessive pyrolysis temperature, NPCA-820 had a lowest N content, which could result in a lower supercapacitive performance. It was also obvious that the capacitance based on the CV curve area decreased in the order of NPCA-650 > NPCA-NaCl-650 > NPCA-n650, which indicated the positive effect of salt templates. Noteworthily, the CV curve area of NPCA-650 was much larger than the other two samples because of the use of NaOH to improve the pore structure, especially the microporosity. Fig. 4b shows the GCD curves of NPCAs at 0.5 A g
1. All the curves showed a quasi triangular-like shape, which confirmed the dominant contribution of double layer capacitance [10]. Small deviations from a linear shape could be observed, also demonstrating the existence of faradaic capacitance, consisting with the CV results [14]. The specific capacitances of the samples calculated from GCD curves at 0.5 A g
1 were 182.7 F g
1 (NPCA-500), 264.3 F g
1 (NPCA-650), 198.4 F g
1 (NPCA-820), 158.5 F g
1 (NPCA-NaCl-650) and 138.2 F g
1 (NPCA-n-650), respectively. NPCA-650 exhibited a much higher specific capacitance than the other samples. This value of specific capacitance was also higher than or comparable with many reported carbons as listed in Table S1. Fig. 4c and d shows the CV curves and GCD curves of NPCA-650 at different scan rates and current densities. The quasi-rectangular shape were observed in all CV curves and inconsiderably distorted even at a high scan rate of 200 mV s
1, implying a fast charge propagation [51]. The GCD curves at various current densities (0.5e20 Ag-1) were nearly isosceles triangle, suggesting a good electrochemical reversibility and high rate capability [52]. The corresponding specific capacitances at different current densities are summarized in Fig. 4e. At a high current density of 20 A g
1, NPCA-650 still offered a specific capacitance of 147.8 F g
1, a retention of 55.9% compared with that at 0.5 A g
1. This value of capacitance retention was comparable with that of NPCA-820 (55.1%) and higher than that of NPCA-500 (27.1%), NPCA-NaCl-650 (37.1%) and NPCA-n-650 (22.1%). The good rate capability could be attributed to the developed pore structure and abundant N dopants [53]. The long-term cyclic stability of NPCA-650 in the three-electrode system was

measured at 5A g
1 (Fig. 4f). The NOCA-650 based electrode showed a good cyclic stability that the capacitance was not obviously decreased after 7000 cycles and still retained 95.89% after 10,000 cycles. Since NPCA-650 exhibited a better electrochemical performance, a symmetric two-electrode system was applied to further evaluate the supercapacitive performance in 1 M Na2SO4 electrolyte as the Na2SO4 electrolyte have been reported to effectively extend the voltage window for a higher energy/power density [6]. The CV curves maintained a quasi-rectangular shape even at a high scan rate (Fig. 5a) and the GCD curves showed nearly symmetrically triangular shape at various current densities of 0.3e20 A g
1, suggesting a good capacitive characteristic. The specific capacitances were also calculated from the GCD curves. As shown in Fig. 5c, the specific capacitance based on the total cell at 0.3 A g
1 was 139.3 F g
1 and still remained at 72.1 F g
1 even at a high current density of 20 A g
1, revealing a good rate performance. The Nyquist plot exhibited a small semicircle at the high frequency region and a linear shape in the low frequency region (Fig. 5d). The nearly vertical straight line suggested a dominance of ideal double-layer charge/discharge behavior [14]. Warburg impedance was observed in the medium frequency region, representing the ion diffusion/transport into the depth of the micro/ mesoporous network of the electrode materials [54]. The small diameter of the semicircle at the high frequency region indicated a low charge transfer resistance (Rct) related to the faradaic reactions for pseudocapactive performance [50]. The intercept at the real axis represented the equivalent series resistance (ESR). The ESR of NPCA-650//NPCA-650 symmetrical supercapacitor was as low as 1.35 U, suggesting a good conductivity property of the electrode [10]. The Ragone plot of NPCA-650//NPCA-650 symmetrical supercapacitor is shown in Fig. 5e. The as-assembled supercapacitor exhibited a high energy density of 12.4 Wh kg
1 at a power density of 120 W kg
1 and it could retained at 6.4 Wh kg
1 at a high power density of 8 kW kg
1, which was higher than that of many reported carbon materials [51,55e61]. indicating a good supercapative

Fig. 5. electrochemical performance of NPCA-650//NPCA-650 symmetric supercapacitor in 1 M Na2SO4 electrolyte: (a) CV curves at various scan rates, (b) GCD curves at various current densities, (c) specific capacitance at different current densities based on the total symmetrical system, (d) Nyquist plot in the frequency range from 0.01 Hz to 10 kHz, (e) Ragone plot and (f) cycling performance at 3 A g
1.

P. Li et al. / Electrochimica Acta 353 (2020) 136514

performance. Fig. 5f shows the long cycle life of the supercapacitor and the specific capacitance could retain at 98.6% after 10,000 cycles, which suggested a negligible loss in capacitive performance, indicating an excellent cycling stability. 4. Conclusions Nitrogen doped hierarchical porous carbon aerogels (NPCAs) have been prepared from self-assembly natural biopolymer gel via a novel dual-template method. NaCl was used to support the carbon structure for 3D architecture and a splash of NaOH was used to improve the porosity. As a result, the high surface area carbon was obtained without serious environmental threats caused by the extensive use of strongly alkaline etching agents. Waste-biomassbased xanthan gum as the precursor could enhance economic efficiency of the biomass waste treatment. It was found that the N element could be retained into the carbon framework although the NaCl and NaOH were applied. It was demonstrated that NaCl was important to form the 3D porous structure. NaOH was in favor of the development of micropores to enhance the specific surface area (SSA). Besides, the pyrolysis temperature showed a significant effect on the structure of NPCAs. High temperature could help enhance the specific surface area but reduced the content of N element. When applied as the electrode material, NPCA-650 exhibited the highest specific capacitance (264.3 F g
1) among the as-prepared samples due to its both developed pore structure and high content of N element. Furthermore, the NPCA-650//NPCA650 symmetric supercapacitor displayed a high energy density of 12.4 Wh kg
1 and an excellent cycling stability (retaining 98.6% after 10,000 cycles). Thus, the NPCAs from biopolymers showed the potential to be a promising candidate for supercapacitor electrode materials. This study provides a novel method to fabricate low-cost, sustainable and high-performance carbon-based energy storage materials. Declaration of competing interest None. CRediT authorship contribution statement Panyu Li: Conceptualization, Methodology, Investigation, Writing - original draft, Writing - review & editing, Funding acquisition. Hongyang Xie: Software. Yali Liu: Formal analysis. Jie Wang: Resources. Xuqian Wang: Data curation. Yi Xie: Visualization. Wanrong Hu: Visualization. Tonghui Xie: Project administration. Yabo Wang: Data curation, Project administration. Yongkui Zhang: Supervision, Project administration. Acknowledgements This work was supported by Graduate Student’s Research and Innovation Found of Sichuan University (2018YJSY078). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.electacta.2020.136514. References [1] R. Ravit, J. Abdullah, I. Ahmad, Y. Sulaiman, Electrochemical performance of poly(3, 4-ethylenedioxythipohene)/nanocrystalline cellulose (PEDOT/NCC) film for supercapacitor, Carbohydr. Polym. 203 (2019) 128e138. [2] Z. Ye, F. Wang, C. Jia, K. Mu, M. Yu, Y. Lv, Z. Shao, Nitrogen and oxygencodoped carbon nanospheres for excellent specific capacitance and cyclic

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